Gas turbine engines of the type used for aircraft propulsion include a fan rotor which comprises an array of fan blades recessed within an engine intake duct and projecting radially outwardly from a central hub. During engine operation, the hub and blades rotate about a longitudinally extending axis to pressurize a stream of air flowing through the duct. At sufficiently high rotational speeds, the radially outermost portions of the fan blades operate supersonically so that the leading edge of each blade generates an aerodynamic shock. These shocks propagate forwardly, i.e. opposite to the direction of flow of the air stream, and exit from the intake duct.
In practice, each blade in the array differs slightly from the other blades in the array due to allowable manufacturing and installation tolerances. Because of these blade-to-blade nonuniformities, the shocks are nonuniformly oriented and propagate at different speeds. As a result, the shocks interact within the intake duct to produce a complex, time varying air pressure pattern which repeats once per rotor revolution. The frequency spectrum of this pressure pattern includes a fundamental frequency of once per fan rotor revolution as well as multiple higher order harmonics of the fundamental frequency. The noise associated with the pressure pattern is known as combination tone noise or multiple pure tone noise and, when it propagates forwardly beyond the confines of the intake duct, can be objectionable to airplane cabin occupants and to residents of communities in the vicinity of airports.
Combination tone noise is likely to become an increasingly prevalent problem since the fans of modern engines have a larger diameter than those of earlier generation engines and therefore operate at slower rotational speeds. In addition, modern engines often employ fewer fan blades than earlier generation engines. Both of these trends, slower rotational speeds and reduced blade quantity, result in the acoustic energy of the combination tone noise being concentrated at lower frequencies. Since acoustic energy at low frequencies is less readily attenuable than acoustic energy at high frequencies, current trends in the gas turbine industry appear likely to exacerbate the problem of combination tone noise.
One possible way to address the problem of combination tone noise is to employ sound attenuating material in the fuselage of the aircraft to shield occupants from the noise. However the sound attenuating material adds cost and weight to the aircraft and can be difficult to incorporate if the need for the material was not anticipated early in the design of the aircraft. Moreover, the use of sound attenuating material in the aircraft does nothing to alleviate the community noise problem.
Another possible way to address combination tone noise is to minimize the magnitude of the blade-to-blade nonuniformities by imposing more stringent manufacturing tolerances. However this approach is undesirable since it escalates the cost of blade manufacture. The adoption of stricter tolerances also fails to remedy combination tone noise generated by an existing fan comprised of blades manufactured prior to the adoption of those tolerances unless the engine owner refurbishes the existing blades or replaces them with newly manufactured blades--expensive and unappealing actions if the existing blades are otherwise serviceable. The unconditional imposition of stricter manufacturing standards may also be undesirable because excessive blade-to-blade uniformity can make the fan susceptible to flutter, a potentially destructive aeroelastic phenomenon.
Minimization of blade to blade nonuniformity might also be achieved by carefully selecting a subset of nearly uniform blades from a larger set of less uniform blades. However this approach relies on the presence of a sufficiently large inventory of blades from which the subset can be selected. An engine manufacturer is unlikely to possess such an inventory, and an engine owner, who may be faced with the occasional need to replace several damaged blades in a rotor, is almost certain not to possess an adequate inventory. Even if such an inventory was available, the requirement to select a nearly uniform subset of blades slows the pace of assembly operations and, without additional selection criteria, introduces the possibility of flutter referred to above.
It is seen, therefore, that existing methods for addressing the problem of combination tone noise are unsatisfactory and that a simple, effective and economical way for mitigating combination tone noise is needed.